What Is HP Multi Jet Fusion and Why Do Design Rules Differ from SLS?
HP Multi Jet Fusion (MJF) is a powder bed fusion process that uses an inkjet printhead to deposit fusing and detailing agents onto a nylon powder bed, which is then fused by an infrared energy source. While MJF and SLS both use PA12 nylon powder, MJF produces parts with a finer surface finish (Ra 5–8 µm vs Ra 10–15 µm for SLS), more consistent mechanical properties, and a distinctive grey appearance. These differences arise from how energy is delivered in MJF — through chemistry rather than a single laser spot — and they affect design rules in several important ways.
If you already design for SLS, most SLS rules carry over to MJF, but there are key differences in achievable feature resolution, surface quality, and colouring behaviour covered in this guide.
Wall Thickness
MJF achieves finer resolution than SLS because the fusing agent is deposited precisely by an inkjet printhead at 1,200 dpi before energy is applied:
- Absolute minimum wall: 0.5 mm — functional but fragile, prone to breakage during depowdering
- Recommended minimum: 1.0 mm for general structural walls
- Load-bearing walls: 2.0 mm or greater
- Large flat panels (> 100 mm): Add ribbing or waffle structure to prevent warpage — flat panels in MJF can warp by 0.3–1.0 mm over 200 mm spans
MJF minimum wall is slightly better than SLS (0.5 mm vs 0.7 mm) because the fusing agent provides more precise boundary definition than a Gaussian laser beam. However, very thin walls in MJF absorb dye unevenly — if cosmetic appearance matters, keep walls above 1.5 mm for consistent dyeing results.
Tolerances and Dimensional Accuracy
| Dimension | MJF Tolerance | SLS Comparison |
|---|---|---|
| First 100 mm | ±0.3 mm | ±0.3 mm (same) |
| Beyond 100 mm | ±0.3% of dimension | ±0.3% (same) |
| Hole diameter | Undersized by 0.2–0.4 mm | Undersized by 0.3–0.5 mm (MJF better) |
| Fine features (< 2 mm) | ±0.2 mm | ±0.3 mm (MJF better) |
| Surface roughness (as-built) | Ra 5–8 µm | Ra 10–15 µm (MJF better) |
MJF parts cool more uniformly than SLS because the entire build volume is at processing temperature (not just near a laser spot). This reduces thermal gradients and produces slightly better dimensional consistency, especially for larger parts.
Holes, Pins and Clearances
- Minimum hole diameter: 1.5 mm (MJF resolves smaller holes than SLS)
- Hole sizing compensation: Add 0.3 mm to nominal diameter for clearance fits; 0.1 mm for press fits
- Minimum pin diameter: 1.0 mm (isolated pins < 1.5 mm diameter are fragile in depowdering)
- Moving parts (living hinges, snap fits, assemblies): Leave 0.3 mm clearance between mating surfaces to prevent fusing during the build
- Printed-in-place assemblies: MJF can print assembled kinematic linkages, hinges, and chains with 0.3 mm clearance — test specific geometries with a small run first
Hollow Parts and Powder Removal
MJF uses unfused powder as support material — hollow sections must allow powder to escape after printing. Unlike SLS where parts can sometimes be gently vibrated to remove powder, MJF uses a dedicated powder removal station with vacuum extraction. Design rules:
- Any internal cavity must have at least one escape hole ≥ 5 mm diameter
- For multiple cavities, use escape holes at both ends to allow through-flow of compressed air during cleaning
- Blind holes deeper than 30 mm should have escape holes at the bottom — fine powder packs tightly in deep blind features
- Gyroid and open-cell lattice structures drain naturally — no escape holes needed
Surface Finish and Dyeing Characteristics
MJF parts print in a natural grey colour with a distinctive matte, slightly grainy texture. The grey is caused by the carbon-based fusing agent absorbed into the part surface. This creates two unique design opportunities:
- Dyeing: MJF parts dye exceptionally well because the grey base and porous surface absorb dye uniformly. Black is the most common, but custom colours are possible. Thin walls dye through completely; thick walls dye to about 1–2 mm depth from the surface.
- Painting: Light sanding (320–400 grit) produces a smooth paintable surface suitable for consumer products
- Vapour smoothing (PA12): Chemical vapour smoothing of MJF parts reduces Ra from 5–8 µm to 1–2 µm, approaches injection moulded finish quality
MJF vs SLS: Design Rule Summary
| Design Aspect | MJF | SLS |
|---|---|---|
| Minimum wall | 0.5 mm | 0.7 mm |
| Minimum hole | 1.5 mm | 2.0 mm |
| Surface finish (as-built) | Ra 5–8 µm | Ra 10–15 µm |
| Powder removal | Requires active extraction | Manual depowdering acceptable |
| Colour options | Grey base, excellent dyeing | White/beige, less even dyeing |
| Isotropy | Fully isotropic | Fully isotropic |
| Snap fits / living hinges | Excellent | Excellent |
| Cost per kg | Slightly lower (HP faster throughput) | Slightly higher |
For most PA12 applications, MJF is the preferred choice at Layer X due to better surface finish and finer feature resolution. Get an MJF quote to compare with SLS for your specific geometry.